Composite Wafer Having a SiC-Based Functional Layer

ABSTRACT

A composite wafer includes a substrate and a SiC-based functional layer. The substrate includes a porous carbon substrate core and an encapsulating layer encapsulating the substrate core. The SiC-based functional layer comprises, at an interface region with the encapsulating layer, at least one of: a carbide and a silicide formed by reaction of a portion of the SiC-based functional layer with a carbide-and-silicide-forming metal. An amount of the carbide-and-silicide-forming metal, integrated over the thickness of the functional layer, is 10 −4  mg/cm 2  to 0.1 mg/cm 2 .

TECHNICAL FIELD

Embodiments described herein relate to wafers, in particular compositewafers having a substrate and a SiC-based functional layer arranged onthe substrate and to methods for manufacturing such wafers.

BACKGROUND

SiC based semiconductor devices offer a number of advantages withrespect to the more common devices made from silicon wafers. Forexample, SiC, which withstands high temperatures and has a wide bandgap,is well-suited for applications in high temperature electronics, such aspower electronic devices or high-temperature sensors.

Due to the high cost of SiC, thin SiC based functional layer isdesirable for the SiC based devices. The functional layer can bearranged on a substrate which provides the bulk material and allows forsufficient mechanical and thermal stability during processing of the SiClayer and possibly also in the final device. In addition to being lessexpensive than SiC, the substrate should adhere well to SiC, should beeasy to handle and able to resist the processing conditions such as hightemperatures, and should not contaminate the processing equipment withunwanted substances.

SUMMARY

According to an embodiment of the invention, a composite wafer comprisesa substrate, comprising a porous carbon substrate core and anencapsulating layer, the encapsulating layer encapsulating the substratecore in an essentially oxygen-tight manner; and a SiC-based functionallayer bonded on the substrate. The SiC-based functional layer comprises,at an interface region with the encapsulating layer, at least one of acarbide and a silicide formed by reaction of a portion of the SiC-basedfunctional layer with a carbide-and-silicide-forming metal. The amountof the carbide-and-silicide-forming metal, integrated over the thicknessof the functional layer, is 10⁻⁴ mg/cm² to 0.1 mg/cm².

According to an embodiment of the invention, a wafer comprises aSiC-based functional layer bonded on the substrate, wherein theSiC-based functional layer comprises on one side at least one of acarbide and a silicide formed by reaction of a portion of the SiC-basedfunctional layer with a carbide-and-silicide-forming metal. The amountof the carbide-and-silicide-forming metal, integrated over the thicknessof the functional layer, is 10⁻⁴ mg/cm² to 0.1 mg/cm².

According to an embodiment of the invention, a method for manufacturinga composite wafer is provided. The method comprises: providing a porouscarbon substrate core; encapsulating the substrate core using anencapsulating layer, thereby obtaining a substrate; providing aSiC-based functional layer; forming an adhesion layer comprising acarbide-and-silicide-forming metal on the SiC-based functional layer oron a portion of the encapsulating layer, the adhesion layer having athickness between 1 nm and 10 nm or between 1 nm and 100 nm; positioningthe SiC-based functional layer on the substrate in such a manner thatthe adhesion layer is interposed between the encapsulating layer and thefunctional layer; and bonding the SiC-based functional layer on thesubstrate in such a manner that at least a portion of thecarbide-and-silicide-forming metal of the adhesion layer reacts with aportion of the SiC of the functional layer to form at least one of acarbide and a silicide. In the composite wafer, the encapsulating layerencapsulates the substrate core in an essentially oxygen-tight manner.

According to an embodiment of the invention, a composite wafercomprises: a substrate comprising a porous carbon substrate core and anencapsulating layer, the encapsulating layer comprising reactivelyformed SiC and encapsulating the substrate core in an essentiallyoxygen-tight manner; and a SiC-based functional layer bonded on thesubstrate. The SiC-based functional layer comprises, at an interfaceregion to the encapsulating layer, at least one of a carbide, asilicide, and a mixture of both.

According to an embodiment of the invention, a method for manufacturinga composite wafer is provided. The method comprises: providing a porouscarbon substrate core; forming a Si layer on the substrate core, andforming reactively an SiC layer from the Si layer, such that thesubstrate core is encapsulated in an encapsulating layer that comprisesthe SiC layer and encapsulates the substrate core in an essentiallyoxygen-tight manner, thereby obtaining a substrate; providing aSiC-based functional layer; forming an adhesion layer comprising acarbide-and-silicide-forming metal on the SiC-based functional layer oron a portion of the encapsulating layer; positioning the SiC-basedfunctional layer on the substrate in such a manner that the adhesionlayer is interposed between the encapsulating layer and the functionallayer; and bonding the SiC-based functional layer on the substrate insuch a manner that at least a portion of thecarbide-and-silicide-forming metal of the adhesion layer reacts with aportion of the SiC of the functional layer to form at least one of acarbide and a silicide.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The components in the figures are not necessarily to scale or includeall details. Instead, the figures are schematic and have the purpose ofillustrating the principles of the invention. Moreover, in the figures,like reference numerals designate corresponding parts. In the drawings:

FIG. 1 is a schematic side view of a composite wafer according to anembodiment;

FIGS. 2 and 3 are schematic side views of composite wafers according tofurther embodiments;

FIGS. 4a, 4b and 5a to 5c are schematic side views of composite wafers,in which the substrate has been fully or partially removed, according tofurther embodiments; and

FIGS. 6a to 6e illustrate a method for manufacturing a composite waferaccording to a further embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and which show, by wayof illustration, specific embodiments in which the invention may bepracticed. In this regard, directional terminology, such as “top”,“bottom”, “front”, “back” etc., is used with reference to theorientation of the Figure(s) being described. Because components ofembodiments can be positioned in a number of different orientations, thedirectional terminology is used for the purpose of illustration and isin no way limiting. It is to be understood that other embodiments may beutilised and structural or logical changes may be made without departingfrom the scope of the present invention. Therefore, the followingdetailed description is not to be taken in a limiting sense, and thescope of the present invention is defined by the appended claims. Theembodiments being described use specific language, which should not beconstrued as limiting the scope of the appended claims.

It is to be understood that features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise. For example, features illustrated ordescribed as part of one aspect or embodiment can be used in conjunctionwith features of other aspects or embodiments to yield yet a furtheraspect or embodiment. It is intended that the present descriptionincludes such modifications and variations.

The term “composite wafer” relates to any wafer having a (hereSiC-based) functional layer and an additional element within or outsidethe functional layer such as an interface region. In particular, theterm “composite wafer” refers to a wafer having a functional layer and asubstrate on which the layer is bonded. In alternative embodiments, thesubstrate may have been removed, leaving behind only an interfaceregion.

The term “vertical” as used in this specification intends to describe anorientation which is arranged perpendicular to the main surface of thesemiconductor substrate.

The term “functional layer” as used in this specification intends todescribe a layer that contributes directly to the functionality of asemiconductor device, such as a sensor, a diode and/or a transistor, dueto its semiconducting properties, possibly after additional doping,layering, structuring, or other kinds of processing of the functionallayer. When referring to semiconductor devices, normally at leasttwo-terminal devices are meant, one example thereof being a diode.Semiconductor devices can also be three-terminal devices such as afield-effect transistors (FET), insulated gate bipolar transistors(IGBT), junction field effect transistors (JFET) and thyristors, to namea few. The semiconductor devices can also include more than threeterminals. According to an embodiment, semiconductor devices are powerdevices.

The term “SiC-based” layer as used in this specification intends todescribe a layer including Silicon carbide (SiC), i.e. a compound ofsilicon and carbon. This does not exclude the presence of otherelements. Herein, a SiC based functional layer is understood to comprisemainly SiC (more than 50% SiC, preferably more than 80% SiC) in anycrystal configuration, e.g. in a stacked configuration. In thisdocument, any element concentration or ratio is based on particle number(number of atoms or molecules) unless stated otherwise.

The term “carbide forming metal” as used in this specification intendsto describe a metallic element capable of forming a carbide whenreacting with C, in particular when being brought to reaction with theSiC-based layer. The carbide forming metals can also be present in acompound, especially in a compound containing nitrogen N, such as e. g.titanium nitride. The reaction may only take place at high temperatures,e.g. above 700° C. The resulting carbide is then stable (also at lowertemperatures). Likewise, the term “silicide forming metal” intends todescribe a metal capable of forming a silicide when reacting with Si, inparticular when being brought to reaction with the SiC-based layer.Materials included in this definition are e.g. transition metals fromone of groups 4 to 10 of the periodic table. Specific metals having thisproperty include Mo, Ta, Nb, V, Ti, W, Ni, and Cr. The silicide formingmetals can also be present in a compound, especially in a compound withSi, such as silicium molybdenum. The term “amount of thecarbide-and-silicide-forming metal” includes the amount of the metalpresent in any compound, and in particular includes the amount of themetal present in carbide and silicide of the SiC-based layer.

The term “oxygen-tight” as used in this specification intends todescribe a material that is essentially oxygen-tight under processingand usage conditions of the electronic component (e.g. at temperaturesup to 1500° C. or 1550° C.). “Essentially oxygen-tight”, as used herein,is understood as only allowing a negligible amount of oxygen, e.g. lessthan 1 mg O₂/cm²/h, possibly even less than 0.1 mg/cm²/h.

The term “encapsulating” layer as used in this specification intends todescribe a layer that coats the substrate core from all sides. The layercan include different sub-layers, e.g. one sub-layer for each side orone sub-layer arranged outside the other sub-layer. Also,“encapsulating” implies that the encapsulating layer (sub-layers) haveno fully open gaps.

The term that the functional layer is “bonded” on the substrate includesany connection obtainable by bonding, in particular by thermal bonding,and does not exclude an additional layer between the two layers such asa bonding-material layer, although it is preferred that the functionallayer directly contacts the substrate. Here, the term “directlycontacts” means that the functional layer is directly adjacent to thesubstrate without any continuous other layer in between (e.g. nocontinuous layer of non-reacted carbide-and-silicide-forming metal).This does not exclude some local impurities or islands of non-reactedmetal or other material present at their interface, for example carbideand/or silicide islands. Preferably such islands occupy less than 20% ofthe interface surface.

Specific embodiments described herein pertain to, without being limitedthereto, composite wafers including a substrate and a SiC-basedfunctional layer, wherein the substrate includes a porous carbonsubstrate core and an encapsulating layer (oxygen barrier layer)encapsulating the substrate core, wherein the SiC-based functional layerhas been bonded on the substrate by reacting a thin adhesion layer ofcarbide-and-silicide-forming metal as a bonding layer. Thecarbide-and-silicide-forming metal thereby forms a carbide and/orsilicide which chemically reacts with the SiC-based functional layer.The adhesion layer is so thin (thickness between 1 nm and 10 nm) thatits material typically completely reacts with the SiC to form carbidesand silicides which then adhere to the SiC layer. Nevertheless, due tothe thinness of the adhesion layer, the metal (e.g. unreacted or boundin carbide and/or silicide) is present in such a small amount (less than10⁻⁴ mg/cm² to 0.1 mg/cm²) that a diffusion into the SiC layer is verylimited, so that the metal does not diffuse significantly into the SiCbulk material but is essentially limited to an interface layer.Consequently, the low amount of metal does not interfere negatively withthe function of the SiC layer. On the other hand, it has beensurprisingly discovered that even if the adhesive layer is so thin thatit completely reacts upon bonding, it still has a strong adhesiveeffect, similar to the adhesive effect of a thicker adhesive layer thatwould, however, lead to more diffusion of metal into the SiC layer.Other embodiments described herein pertain to wafers as described above,but from which the substrate has been fully or partially removed.

With reference to FIG. 1, a first embodiment of a composite wafer 10 isdescribed. Substrate 11 includes a porous carbon substrate core 12 andan encapsulating layer 14. The encapsulating layer acts as an oxygenbarrier and encapsulates the substrate core 12 in an essentiallyoxygen-tight manner.

Further, a SiC-based functional layer 18 is arranged (bonded) on thesubstrate 11. In the functional layer 18, an interface region 17 can beseen at an interface with the encapsulating layer 14. The interfaceregion 17 contains carbides and/or silicides formed by reaction of aportion of the SiC-based functional layer with acarbide-and-silicide-forming metal, and optionally also comprises someremaining non-reacted carbide-and-silicide-forming metal. In thisembodiment, the amount of the carbide-and-silicide-forming metal(present in the carbides and silicides and optionally also innon-reacted form) is 10⁻⁴ mg/cm² to 0.1 mg/cm², integrated over thethickness of the functional layer, i.e. integrated in directionperpendicular to a plane of the interface between the encapsulatinglayer 14 and the functional layer 18.

By using a substrate 11 with a porous carbon core 12, the mass of thesubstrate 11 can be kept small due to the small mass density of theporous carbon. This is a significant advantage over other substratesthat have similarly good adherence such as substrates from pure Mo. Thedensity of Mo is relatively high (10.2 g/cm³), and a Mo substrate istherefore difficult to handle and transport due to its weight.Commercially available semiconductor processing equipment, which hasbeen optimized for the mass of Si wafers, in many cases may not handleand transport a Mo substrate reliably. The same applies to substratesmade from other metals. In contrast, the composite wafer 10 shown inFIG. 1 can be handled using standard process equipment.

However, a carbon carrier is very sensitive to oxygen, especially athigh temperatures, e.g., during a furnace process. If the carbon reactswith oxygen (burns), the resulting CO₂ may expand and lead to thebreaking off of protective layers. This problem is solved by theencapsulating layer 14 that protects the carbon substrate core 12 fromoxygen. Another advantage of the porous carbon core 12 is that itadheres well to a large variety of materials, so that the encapsulatinglayer 14 generally adheres well to the carbon core 12.

The composite wafer of FIG. 2 is analogous to that of FIG. 1 exceptwhere noted otherwise. Namely, the composite wafer of FIG. 2 differsfrom that of FIG. 1 in that an additional adhesion layer 15 isinterposed between the encapsulating layer 14 and the functional layer18. Thereby, the adhesion may be further improved. The adhesion layer ispreferably made of a high-temperature-resistant material with lowdiffusion into the functional layer. For example, the adhesion layer 15may be made of SiC having a different crystal structure from thefunctional layer and or from the encapsulating layer 14.

The composite wafer of FIG. 3 is analogous to that of FIG. 1 exceptwhere noted otherwise. Namely, the wafer of FIG. 3 further comprises asoldering portion 20 on the bottom side of the substrate 11, i.e. theside opposite to the functional layer 18. The soldering portion forms anelectrical contact with the SiC functional layer 18 via the core 12 orvia the encapsulating layer 14. To this purpose, at least a portion ofthe encapsulating layer 14 connecting the top side of the substrate 11(the side facing the functional layer 18) and the bottom side of thesubstrate 11 (the side opposite to the functional layer 18) iselectrically conductive. Herein, electrically conductive is defined ashaving a resistivity of less than 10⁻³ Ω·m.

In the following, some general aspects of the invention are discussedwith reference to FIGS. 1 to 3. Herein, these Figures serves asillustration, but it is understood that each of the general aspects canalso be realized in other embodiments than that of FIGS. 1 to 3,optionally in combination with any other general aspect.

Firstly, some general aspects of the SiC-based functional layer 18 arediscussed. The SiC-based functional layer 18 mainly includes a compoundof silicon and carbon. This does not exclude the presence of otherelements, e.g. in the case of a doped layer or of other layers or ofdiffused metal, but according to an aspect, the combined content of Siand C in the layer is 80% or more. According to an aspect, the SiC layermay even be an essentially pure SiC layer, i.e. with a combined Si and Ccontent of 99% or more. Preferably the functional layer consists of pureSiC, carbide, silicide, non-reacted carbide-and-silicide-forming metal,and at most 1% dopants and/or impurities. The ratio of Si to C ispreferably, but not necessarily, about 1:1, e.g. between 0.9 and 1.1.Generally the SiC compound has a layered crystal structure but it mayhave any other crystal structure of SiC.

According to a further aspect, the functional layer has a thickness ofat least 1 μm or between 5 μm and 20 μm.

According to a further aspect, the functional layer may be a portion ofa power semiconductor device on the basis of SiC, such as a diode,J-FET, IGBT, MOSFET or the like. According to a further aspect, thefunctional layer may be a portion of a high-temperature semiconductordevice such as a high-temperature sensor.

According to a further aspect, the SiC based functional layer has beensplit from a SiC wafer by proton-induced splitting, which is visiblefrom the splitting surface on the side of the functional layer oppositeto the substrate 11, 21, and/or from traces of the protons implanted inthe substrate.

Next, some general aspects of the carbide-and-silicide-forming metal andof the interface region 17 are discussed. According to an aspect, theinterface region 17 contacts directly the encapsulating layer withoutany further continuous layer in between. In particular, there is nocontinuous layer of non-reacted carbide-and-silicide-forming metal.However, this does not exclude some local impurities of non-reactedmetal as long as they do not form a continuous layer.

According to an aspect, the amount of the carbide-and-silicide-formingmetal, integrated over the thickness of the functional layer 18, is 10⁻⁴mg/cm² to 0.1 mg/cm². According to a further aspect, the amount of thecarbide-and-silicide-forming metal is mainly concentrated at the side ofthe substrate 11 (at the interface region 17), with more than 50%,preferably more than 80% of the carbide-and-silicide-forming metal beingpresent in the interface region. According to an aspect, the interfaceregion has a thickness of 300 nm or less. (Before the process, thethickness is preferably even less than 100 nm).

The forming of carbides and silicides from suitable metal in theinterface region 17 has the advantage of ensuring good adherence of thefunctional layer 18 to the substrate 11. In particular, the interfaceregion 17 may be formed by a reaction of metals of a thin metal layerwith Si and C from the functional layer 18 to form carbides and/orsilicides, possibly after a high-temperature treatment. This reactionensures a particularly good adherence, regardless of whether the SiClayer is contacted at its C face (so that mainly carbides are formed) orif the SiC layer is contacted at its Si face or at a mixed face, e.g. inthe case of an amorphous or polycrystalline SiC layer 18. Thereby,according to an aspect, the adherence between the substrate 11 and thefunctional layer 18 is higher than 5 to 10 MPa, Additionally oralternatively, the adherence may be even stronger than an adherencewithin the carbon layer 12, so that when the bonded functional layer ispulled off with strong force, the carbon layer is fractured rather thanthe interface between the substrate 11 and the functional layer 18.

According to a further aspect, the carbide and silicide forming metal isa transition metal from one of groups 4 to 10 of the periodic tablehaving this property. For example, the carbide and silicide formingmetal may include, or be, at least one element selected from the groupconsisting of: Mo, Ta, Nb, V, Ti, W, Ni, and Cr, Ti, Mo and W areespecially advantageous due to their high temperature strength. Furthersuitable materials are metal-silicon bilayers or other metal compoundscapable of forming carbide and silicide.

According to a further aspect, during the bonding of the SiC-basedfunctional layer 18 on the substrate 10, the interface region 17 ensuresnot only that the adhesion is strong, but also that the crystalstructure is not transferred, so that no defects are induced in thefunctional layer. Hence, it is advantageous that the interface region 17has a crystal structure that is different from that of the functionallayer 18.

According to a further aspect, the interface region 17 includes aplurality of different intermediate layers. In particular, theintermediate layers contain reaction products of the carbide andsilicide forming metal with the SiC based functional layer, e.g. atleast one carbide phase and/or at least one silicide phase. For example,in the case of the carbide-and-silicide-forming metal being Mo, thephases may include one or more of MoCSi, MoSi, and MoC phases.Generally, these phases can be obtained by only moderately heating thecomponents (to less than 700° C., e.g. in the range 500-700° C.), andthe resulting carbide phase and/or silicide phase are neverthelessgenerally highly temperature resistant and well-suited to the furtherprocessing steps and working conditions even at high temperature.

According to an aspect, the interface region 17 is electricallyconductive, and in particular has a resistivity of less than 10⁻³ Ω·m.

Next, some general aspects of the encapsulating layer 14 are discussed.According to an aspect, the encapsulating layer 14 comprises (may bemade of or have a sub-layer made of) at least one of SiC, a Si oxide, aSi, a Ti oxide, and nitrides like Si₃N₄ or a metal nitride like e.g. TiNor TaN. According to a preferred aspect, the encapsulating layer (or asub-layer thereof) is made of Si that has been reactively obtained froma Si layer.

According to a further aspect, the encapsulating layer 14 may be amulti-layered structure including a plurality of sub-layers. Forexample, a first sub-layer of the encapsulating layer may be a layer ofSiC as described herein, and a second sub-layer may be a Si₃N₄ layer.The sub-layers may be arranged next to each other so that theyencapsulate the substrate core 12 jointly, and or they may be arrangedon top of each other, e.g. the second sub-layer being arranged at anouter side of the first sub-layer. In this case, one of the sub-layersmay be an oxygen barrier sub-layer (e g. SiC) and the other sub-layer(s)may have another function, e.g. improving adherence or chemicalinertness.

According to an aspect, the encapsulating layer 14 is temperatureresistant up to temperatures of at least 1500° C. According to anaspect, the encapsulating layer 14 is essentially oxygen-tight fortemperatures of up to at least 1500° C. According to a further aspect,the encapsulating layer has a thickness of at least 300 nm. According toan aspect, the encapsulating layer 14 has a temperature expansioncoefficient differing from that of the functional layer 18 by less than15%.

Next, some general aspects of the substrate 11, 21 are discussed,According to an aspect, the carbon substrate core 12 has a mass densityof at most 5 g/cm³, more preferably of at most 3 g/cm³. According to afurther aspect, the substrate (substrate core including the adheringlayer) has a mass density (i.e. total mass divided by total volume) ofat most 5 g/cm³. According to another aspect, the porosity of thegraphite core is 5% or more, 8% or more, or even 10% or more. Hence, dueto the porosity, the density of the carbon substrate core 12 may even beless than the normal density of graphite (about 2 to 3 g/cm³).

According to an aspect, the substrate core 12 has an average porediameter of at most 30 μm. Typical Pore size is between 5 and 25 μm.There are also other materials/manufactuers. According to an aspect, atleast some of the pores at the surface of the carbon substrate core 12are closed by a pore-plug material. Thereby, the surface of thesubstrate core 12 is smoothened and adherence to the encapsulating layer14 is improved.

According to a further aspect, the substrate core has at least one ofthe following dimensions: a thickness of at least 300 μm or at least 600μm and/or of at most 2 mm or at most 1 mm. The substrate core may havetwo parallel faces separated by the thickness. The faces may be ofsubstantially circular shape. The substrate core's diameter maycorrespond to the diameter of commercially available silicon wafers,such as about 100 mm, 150 mm, 200 mm, 300 mm or 450 mm in order to fitto available equipment for semiconductor processing (herein, “about” isdefined as “up to a deviation of 5%”). Other diameters are alsopossible. According to a general aspect, the diameter is between 80 mmand 600 mm. In other examples, the substrate core's shape may becircular, elliptical, polygonal or rectangular, and/or have a differentdiameter than mentioned above.

It is noted that the composite component or any part thereof, such asthe substrate 10, may also include further layers in addition to thelayers mentioned herein. For example, the functional layer may includeadditional layers such as a buried insulating layer and/or at least oneprotective layer for protecting the functional layer.

FIGS. 4a and 4b shows a wafer according to a further embodiment havingonly the functional layer 18 of FIG. 1. However, the substrate 11 ofFIG. 1 is removed. Thus, the SiC-based functional layer 18 comprises onone side (bottom side with the interface region 17) at least one of acarbide and a silicide formed by reaction of a portion of the SiC-basedfunctional layer with a carbide-and-silicide-forming metal, and theamount of the carbide-and-silicide-forming metal, integrated over thethickness of the functional layer, is 10⁻⁴ mg/cm² to 0.1 mg/cm². Thefunctional layer 18 may also comprise some remaining non-reactedcarbide-and silicide-forming metal, however according to a particularaspect the wafer is free of any continuous layer of non-reactedcarbide-and-silicide-forming metal. In a particular aspect, removaltraces from removing the substrate (e.g. abrasion traces) are detectableon the bottom side of the interface region 17.

In addition, in the embodiment of FIG. 4b , also some abraded SiCmaterial 14′, which is a portion of the former encapsulating layer 14 ofFIG. 1, can be seen. In a particular aspect, removal traces fromremoving the remainder of the substrate (e.g. abrasion traces) aredetectable on the bottom side of the material 14′. The crystal structureof the material 14′ is, in a particular aspect, different from thecrystal structure of the functional layer 18.

The carbide-and-silicide-forming metal may be as described in relationto FIG. 1, e.g. Mo, Ta, Nb, V, Ti, W, Ni, and/or Cr. Also the otherdescriptions of embodiments and aspects illustrated in FIGS. 1-3 applyinsofar as they do not contradict FIGS. 4a and 4 b.

FIGS. 5a and 5b correspond to FIGS. 4a and 4b as described above, withthe following difference. Additionally, the wafer comprises a solderingportion 20 in electrical contact with the SiC functional layer 18. Thesoldering portion is arranged on the side of the wafer of the interfaceportion 17. The soldering portion is in electrical contact with the SiCfunctional layer 18 via the interface portion 17.

The composite wafer of FIG. 5c is analogous to that of FIG. 2 except forthe following differences: A portion of the substrate 11 of FIG. 2 isremoved, whereas a portion of the substrate (portion of substrate core12 and SiC material 14′ from the former encapsulating layer 14 of FIG.2) remains.

Further, the wafer of FIG. 5c further comprises a soldering portion 20on the bottom side of the substrate 11, i.e. the side opposite to thefunctional layer 18. Thereby the soldering portion 20 covers, in anessentially oxygen-tight manner, the side of the substrate core 12 thatwas left exposed when the portion of the substrate was removed. Hence,the material 14′ (first sub-layer comprising e.g. reactively formed SiC)and the soldering portion 20 (second sub-layer) constitute anencapsulating layer 14 that encapsulates the substrate core 12. Further,the soldering portion is in electrical contact with the SiC functionallayer 18 via the core 12 or via the material 14′ of the encapsulatinglayer 14 analogous to the embodiment of FIG. 3 described above.

In an alternative embodiment, the soldering portion 20 of FIG. 5c isreplaced by a two-layer structure with a first sub-layer being anessentially oxygen-tight layer (e.g. of SiC) and a second sub-layer,arranged below (outside) the first sub-layer, being the solderingportion. The resulting wafer is similar to that of FIG. 3, but withremoval traces on the lower side of the substrate core 12 and with theencapsulating layer 14 being composed of two sub-layers (layer 14′ ofFIG. 5c and first sub-layer as described above).

Next, with reference to FIGS. 6a to 6c , a method for manufacturing acomposite wafer according to a further embodiment is described. As shownin FIG. 6a , a porous carbon substrate core 12 is provided. Then, asshown in FIG. 6b , the substrate core 12 is encapsulated using anencapsulating layer 14.

The encapsulating layer can be applied using any method such assputtering, galvanization, CVD deposition, any other method of applyinga layer, or a combination thereof. Possibly, more than one layering stepis performed in order to encapsulate the substrate core 12 from allsides. Optionally, additional process steps such as chemically reactingthe layer are performed for increasing the oxide-tightness of theencapsulating layer 14. For example, the encapsulating layer 14 may beformed as an amorphous or polycrystalline Si layer, and at a later step(possibly after bonding described below) may be reacted to form a SiClayer. In an alternative embodiment, the Si layer may be subjected to areaction with O₂, whereby an essentially oxygen-tight SiO₂ layer isformed. Optionally, the Si layer and/or the encapsulating layer 14 maybe planarized, especially at a face on which the functional layer is tobe bonded. As a result of any of these techniques, an essentiallyoxygen-tight encapsulating layer 14 is obtained (possibly afteradditional process steps such as bonding).

According to a particularly preferred general embodiment of theinvention, the encapsulating layer is made of reactively formed SiC,which was reacted from a Si layer having at least a portion which wasplanarized prior to the reactive forming of the SiC, Such anencapsulating layer can be distinguished from a directly formed SiClayer because a planarization of Si (which is relatively soft) leavesdifferent traces than a planarization of SiC. By planarizing the initialSi layer before the reaction to SiC takes place, the more difficultplanarization of SiC before bonding may be avoided or at least reduced.

As a further method step as shown in FIG. 6c , a SiC-based functionallayer 18 is provided. This can be done before, after, or in parallel tothe process steps shown in FIGS. 6a and 6b . Then, an adhesion layer 16is formed on the functional layer 18 using any layering method such assputtering, galvanization, CVD deposition, or the like, such that theadhesion layer 16 (directly) contacts the SiC-based functional layer 18.The adhesion layer 16 includes a carbide and silicide forming metal andhas a thickness between 1 nm and 10 nm.

Then, as shown in FIG. 6d , the SiC-based functional layer 18, on whichthe adhesion layer 16 has been formed, is arranged on the substrate 11in such a manner that the adhesion layer 16 is interposed between theencapsulating layer 14 and the functional layer 18.

In an alternative embodiment, the adhesion layer may be formed on a topportion of the encapsulating layer 14, instead of being formed on theSiC-based functional layer as shown in FIG. 6c . In this alternativeembodiment, the SiC-based functional layer 18 is then arranged on thesubstrate 11 in such a manner that the adhesion layer 16 is interposedbetween the encapsulating layer 14 and the functional layer 18, i.e. theconfiguration of FIG. 5d is obtained. The remaining steps, explainedbelow, are the same for both embodiments.

In an alternative embodiment, a part of the adhesion layer may be formedon a top portion of the encapsulating layer 14, and a part of theadhesion layer may be formed on the SiC-based functional layer as shownin FIG. 6c . The both parts of the adhesion layer 16 may be of the sameor of different material. In this alternative embodiment, the SiC-basedfunctional layer 18 is then arranged on the substrate 11 in such amanner that the two parts of the adhesion layer 16 are interposedbetween the encapsulating layer 14 and the functional layer 18, i.e. theconfiguration of FIG. 5d is obtained. The remaining steps, explainedbelow, are the same for all three embodiments and variations thereof.

As shown in FIG. 6e , a bonding reaction is then carried out. Thebonding reaction may be a thermal treatment, wherein the componentsshown in FIG. 6d are heated to a temperature allowing the carbide andsilicide forming metal of the adhesion layer 16 or the two parts of theadhesion layer 16 to react with the SiC based functional layer 18 attheir interface. For example, the temperature may be in a range of 500°C. to 700° C. As a result of the bonding reaction (thermal treatment),at least one carbide phase and/or at least one silicide phase is formed.

The adhesion layer 16 is so thin that during the bonding reaction,essentially all of the carbide-and-silicide-forming metal of theadhesion layer 16 reacts with some of the SiC of the functional layer 18to form a carbide and/or a silicide, so that essentially no unreactedmetal of the layer 16 remains (except possibly some local islands ofunreacted material or impurities). In other words, no continuous layerof unreacted carbide-and-silicide-forming metal remains between thefunctional layer 18 and the encapsulating layer 14. As a result, theadhesion layer 16 as a continuous layer disappears, and instead, acarbide phase and/or at least one silicide phase is created andpartially that forms an interface region 17 of the functional layer 18.

According to a particular embodiment, if the encapsulating layer 14 wasapplied as a Si layer, the Si layer may be brought to reaction thusforming an oxygen-tight layer during the thermal treatment. For example,the Si layer may be brought to a reaction forming SiC as explainedabove.

In the following, some general aspects of the invention are discussedwith reference to FIGS. 6a-6e . Herein, these Figures only serve asillustration, and the general aspects can also be realized in otherembodiments.

According to an aspect, prior to the encapsulating step, an additionalstep of applying an adhering/pore-plugging material on the substratecore 12 may be included. Additionally or alternatively, the substratecore 12 may be planarized.

According to an aspect, the encapsulating step can be performed usingany one of the following:

-   -   (i) The porous carbon substrate core 12 is encapsulated by        sputtering, galvanizing and/or depositing the encapsulating        material thereon;    -   (ii) The porous carbon substrate core 12 is encapsulated by        sputtering, galvanizing and/or depositing a precursor material,        and then by reacting the precursor material to obtain the        encapsulating layer.

In method (i), the encapsulating layer 14 may be applied as anessentially oxygen-tight layer; in method (ii), the encapsulating layer(the precursor material) may later be subjected to a reaction to becomeessentially oxygen-tight. Method (i) may be used for applying Mo, Ta,Nb, V, Ti, W, Ni, Cr or another suitable carbide and silicide formingmetal. Method (ii) may be used for applying a Si oxide or SiC layer.Here, first a Si layer (e.g. an amorphous or polycrystalline Si layer)may be applied to the substrate core 12 (to which optionally anadditional adhering/pore-plugging material has been applied), and then aSi oxide or SiC layer is formed reactively from the Si layer. Forexample, a SiC layer can be formed using a heating process at 1000° C.to 2000° C., preferably at 1300° C. to 1500° C., in an environment inwhich the Si layer reacts to SiC. The heating time may vary between 2minutes and 2 hours, depending on the desired thickness. The SiC layeris particularly suitable for the embodiment described herein.Optionally, the precursor material may be planarized at least at thesurface which is to contact the adherence layer 16.

According to an aspect, the SiC-based functional layer 18 can beprovided using proton-induced cutting, also referred to as “smart cut”.According to this proton-induced cutting step, the SiC based functionallayer is first provided as part of a SiC based wafer in which protonshave been implanted at high intensity. Then, the SiC wafer is bonded tothe substrate 11 as described herein. Then, a high-temperature processis performed at which the functional layer (to which the substrate 11has been bonded) is split off from the SiC based wafer. Optionally, thetop surface of the functional layer 18 (i.e. the surface opposite to thesubstrate 11) is abraded or otherwise treated. This technique can benoticed from the component by the implanted protons in the functionallayer (and in some cases by the shape of the top surface).

According to an aspect, the method further includes processing the SiCfunctional layer 18, e.g. by one or more processing steps such asepitaxy; doping; etching; isolation of devices from each other;contacting; and/or packaging. For example, the functional layer may beprocessed so that a semiconductor device on the basis of SiC isobtained, such as a diode, J-FET, IGBT, MOSFET, SiC-SOI device or anyother device mentioned herein.

Also, any of the aspects mentioned with respect to FIGS. 1 to 3 may beused for the method described herein.

As an additional optional process step of the method of FIGS. 6a to 6e ,a soldering portion may be formed on a side of the substrate 11 oppositeto the functional layer 18. This step then results in the configurationshown in FIG. 2. The soldering portion 20 may, for example, be formed bygalvanization or electroplating. The soldering portion 20 may, forexample, include copper or a lead-free solder.

According to an aspect, at least part of the encapsulating layer 14 iselectrically conductive, so that at least one conductive path is formedbetween the SiC functional layer 18 and the soldering portion 20 via theencapsulating layer 14. Also, the soldering portion 20 may comprise astack of different materials or layers. Also, the soldering portion 20may have a plurality of soldering contacts, and respective conductivepaths (electrically isolated from each other) may be formed between eachsoldering contact and a respective portion of the SiC functional layer18 via the encapsulating layer 14.

As an additional optional process step of the method of FIGS. 6a to 6e ,an additional adhesion layer 15 may be applied to the substrate 11 orthe functional layer 18, such that in the configuration of FIG. 6d , theadditional adhesion layer 15 is interposed between the substrate 11 andthe adhesion layer 16. The additional layer 15 may, for example, be madefrom SiC having a different crystal structure than the material of theencapsulating layer 14 or being amorphous. As a result the compositewafer of FIG. 3 is obtained.

As an additional optional process step of the method of FIGS. 6a to 6e ,the substrate 11 may be removed again partially or completely, e.g. byabrasion, after the step of FIG. 6d or 6 e, and optionally after furtherprocessing steps for processing of the functional layer 18. As a result,the structure shown in FIG. 4a or 4 b is obtained.

Alternatively, the substrate may be removed only partially. If thesubstrate is removed only partially, a further layer may be applied tothe remaining substrate portion such that the remaining substrateportion is, again, encapsulated in an oxygen-tight manner, e.g. as shownin FIG. 5 c.

As an additional optional process step, the soldering portion 20 may beformed on the side of the composite wafer from which the substrate 11,or portion of the substrate 11, has been removed. The soldering portion20 may be formed by galvanization. The soldering portion 20 may beformed to be in electrical contact with the SiC functional layer 18.With this optional step, the structure shown in FIG. 5a or 5 b can beobtained.

It is to be understood that the features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following claims and theirlegal equivalents.

What is claimed is:
 1. A composite wafer, comprising: a substrate,comprising a porous carbon substrate core and an encapsulating layer,the encapsulating layer encapsulating the substrate core in anessentially oxygen-tight manner; and a SiC-based functional layer bondedon the substrate, wherein the SiC-based functional layer comprises, atan interface region with the encapsulating layer, at least one of acarbide and a silicide formed by reaction of a portion of the SiC-basedfunctional layer with a carbide-and-silicide-forming metal, and whereinthe amount of the carbide-and-silicide-forming metal, integrated overthe thickness of the functional layer, is 10⁻⁴ mg/cm² to 0.1 mg/cm. 2.The composite wafer of claim 1, wherein the interface region directlycontacts the encapsulating layer without any further continuous layer inbetween.
 3. The composite wafer of claim 1 wherein the carbide andsilicide forming metal is at least one element selected from the groupconsisting of: Mo, Ta, Nb, V, Ti, W, Ni, and Cr.
 4. The composite waferof claim 1, wherein the encapsulating layer is essentially oxide-tightfor temperatures up to at least 1500° C.
 5. The composite wafer of claim1, wherein the encapsulating layer comprises at least one of: SiC, a Sioxide, a Si, a Ti oxide, and Si₃N₄.
 6. The composite wafer of claim 1,wherein pores of the porous carbon substrate core are closed by apore-plug material, which is different from and encapsulated by thematerial of the encapsulating layer.
 7. The composite wafer of claim 1,wherein the substrate has a mass density of at most 5 g/cm³.
 8. Thecomposite wafer of claim 1, wherein at least a portion of theencapsulating layer connecting the side of the substrate facing thefunctional layer and a side of the substrate opposite to the functionallayer is electrically conductive.
 9. The composite wafer of claim 8,further comprising a soldering portion on the side of the substrateopposite to the functional layer, the soldering portion being inelectrical contact with the SiC functional layer.
 10. The compositewafer of claim 1, wherein no continuous layer of unreactedcarbide-and-silicide-forming metal remains between the functional layerand the encapsulating layer.
 11. A wafer, comprising a SiC-basedfunctional layer, wherein the SiC-based functional layer comprises, onone side, at least one of a carbide and a silicide formed by reaction ofa portion of the SiC-based functional layer with acarbide-and-silicide-forming metal, and wherein the amount of thecarbide-and-silicide-forming metal, integrated over the thickness of thefunctional layer, is 10⁻⁴ mg/cm² to 0.1 mg/cm².
 12. The wafer of claim11, wherein the wafer is free of any continuous layer of non-reactedcarbide-and-silicide-forming metal.
 13. The wafer of claim 11, whereinthe carbide and silicide forming metal is at least one element selectedfrom the group consisting of: Mo, Ta, Nb, V, Ti, W, Ni, and Cr.
 14. Thewafer of claim 11, further comprising a soldering portion in electricalcontact with the SiC functional layer.
 15. A composite wafer,comprising: a substrate, the substrate comprising a porous carbonsubstrate core and an encapsulating layer, the encapsulating layercomprising reactively formed SiC and encapsulating the substrate core inan essentially oxygen-tight manner; and a SiC-based functional layerbonded on the substrate, wherein the SiC-based functional layercomprises, at an interface region to the encapsulating layer, at leastone of: a carbide, a silicide, and a mixture of both.